1. Field of the Invention
This invention relates to electromagnetic motors, and more particularly, to vibrator type motors of the type having plural oscillating armatures adapted to reduce undesired noise and vibrations.
2. Description of the Prior Art
A vibrator type motor or electromagnetic motor for reciprocally moving a load in response to magnetic and resilient forces is widely used in many small electric hand tools or implements such as dry shavers and hair clippers. The vibrator type motor comprises a magnetic circuit with a stator unit or stationary portion and an armature or movable portion. The stator unit includes a core generally made of laminations of magnetically permeable material providing a low reluctance path for conducting magnetic flux and a coil of insulated windings wound around the core. Magnetic flux is induced in the core in response to an electrical signal coupled to the coil. The core may be U-shaped with free ends terminating in salient stator poles or protrusions with one or more pole faces. The armature is also made of laminations of magnetically permeable material formed to provide salient armature poles with one or more pole faces. The stator unit is immovably mounted on a motor mount or housing while the armature is pivotally mounted on the motor mount so that the salient armature poles have pole faces separated from pole faces of adjacent stator poles by an air gap. Resilient means, such as springs, are arranged to maintain the armature in a preferred rest position where the armature pole faces are angularly displaced from the adjacent stator pole faces. It is well known that the armature and stator pole faces bounding the air gap attract each other when the core is magnetized by coupling an alternating current (AC) signal to the windings. The AC signal induces a magnetic flux in the core and a resulting magnetic field in the air gap which causes the armature to be rotated in a preferred direction against the bias forces provided by the springs, until the stator and armature pole faces are in substantial alignment. When the amplitude of the first half wave of the AC signal is reduced from a peak value to zero, the magnetic field breaks down and the elastic properties of the stressed springs cause the armature to rotate in an opposite direction, back to the neutral or rest position to complete one cycle of armature movement. A succeeding half wave of the AC signal starts the armature moving again toward the position of stator and armature pole face alignment to eventually complete a second cycle of armature movement. Thus, it will be appreciated that the armature oscillates at twice the frequency of the AC input signal in response to a magnetic force having a periodically varying amplitude.
In general, when a periodically varying force is applied to a body mounted on springs or other elastic supports, the body will vibrate. Thus, it will be appreciated that vibratory motors of the type described above tend to produce a reaction force causing excessive noise and external vibrations in the motor and its housing. The frequency of the external vibrations is directly proportional to the frequency of the armature oscillation. The magnitude of the external vibrations depends partly on the moment of inertia of the armature. Attemps to minimize noise and vibrations in a vibratory motor include various arrangements which minimize the moment of inertia of the armature. For example, U.S. Pat. No. 3,493,793, "Hair Clipper Having Oscillating Armature Motor" issued to P. W. Niemela on Feb. 3, 1970, discloses the use of a stator including an E-shaped core cooperating with an armature having a permanent magnet so that the armature oscillates at the frequency of the AC input signal coupled to the stator windings. In addition, the mass of the armature is minimized and means are provided for reversing the direction of armature movement with a minimum of housing vibration. For many vibratory motor applications, merely reducing the oscillating frequency of the armature to that of the AC input frequency and minimizing the mass of the armature would not reduce undesired noise and vibration to an acceptable level.
Other attempts to reduce objectionable noise and vibration to an acceptable level include arranging the vibratory motor to have multiple armatures designed to oscillate in opposite directions to balance out forces producing the noise and vibration. In U.S. Pat. No. 3,218,708, "Electrically Operated Shaver", issued to A. R. Spohr on Nov. 3, 1965, a vibratory motor is provided with multiple coiled springs for positioning a pair of armatures between opposing poles of a stator in a manner that will cause the armatures, and electric shaver cutter heads attached to each armature, to oscillate in opposite directions. However, it is sometimes difficult to achieve perfect balancing of multiple cutter heads. U.S. Pat. No. 3,144,571, "Electromagnetic Motor Having Oppositely Oscillating Armatures", issued to S. R. Kukulski on Aug. 11, 1964, and U.S. Pat. No. 2,299,952, "Vibratory Motor For Dry Shaver And The Like", issued to I. Jepson on Oct. 27, 1942, describe vibratory motors utilizing a driving armature and a secondary armature disposed between opposing poles of a stator. The driving armature is coupled to a load, such as a single cutting head in a dry shaver, and the secondary armature is connected to a counterweight. The vibratory motor is arranged so that the driving and secondary armatures pivotally oscillate in opposite directions to balance out undesired vibration causing forces. However, the secondary armature does not contribute to the force necessary to drive the load resulting in an inefficient use of motor generated energy.
Accordingly, it is desired to provide an efficiently operating vibratory motor arranged to move a load with minimum noise and vibration at any desired frequency of motor operation.